Turbo-machinery, such as steam and gas turbines, include a plurality of blades arranged in rows extending radially from an axially aligned shaft, the rows of blades being rotatable in response to the force of a high pressure fluid flowing axially through the machine. Due to their complex design, the blades have many resonant vibrational frequencies. Blade resonant frequencies which are excited may create stresses sufficient to break the blades and cause extensive damage. Although blades are designed and tested prior to machine installation in order to prevent resonant vibration, such evaluations performed prior to actual use may not subject the blades to the same temperature, pressure, flow, and rotational conditions which are experienced during normal operations. Consequently, it is desirable to monitor rotating blades on-line in order to detect all resonant vibrations. It is also desirable to monitor rotating blades on-line in order to detect new vibration problems which develop after a turbomachine is put in use. Changes in blade vibrations may be indicative of significant structural changes which may also lead to extensive damage as well as costly down time while the machine undergoes repair.
Vibration monitoring systems have been developed for on-line detection of blade vibrations. A typical system employs non-contacting proximity sensors concentrically mounted about individual blade rows. Each sensor is used to detect motion of a rotating blade tip about its normal position in a rotating time frame. Each of the sensors, which may be of the magnetic induction, capacitive, or optical type, develops an electrical output as each blade tip rotates past it. Many vibration monitoring systems measure the time required for a blade tip to travel between two reference points which are separated by a known distance. Deviations between the measured travel time of the blade tip and the expected travel time based on shaft rotational speed are calculated. The deviations are used to reconstruct a time history of vibrational movement for each blade tip. Fourier analysis is then used to determine the amplitudes and frequencies of the vibration present in each blade.
In the past, steam turbines have been shut down and cooled and, for any but the last row of blades, disassembled to provide access to the sensors. A probe installation commonly used on turbines includes using a guide tube sealed at the cylinder penetrations of the turbine and having a gate valve, for measuring the pressure in the flow path of the turbine. Sensors in these cases have been mounted externally of the guide tube and not in close proximity to the rotating blades. In the present invention, a method of probe installation includes a guide tube, gate valve, and seals at the turbine cylinder penetrations. The sensor is mounted in the probe tip rather than externally of the turbine cylinder and is located in a precise spaced relationship with the rotating blade tips of the turbine and is positioned radially by a shoulder near the bottom of the tube. Probes may be accurately positioned while the rotary machine remains in operation.